These are Plutonic rocks, which are volcanic rocks that are formed and cool at depth within the earth. They are only stable under high pressures so when they weather out to the surface, where there is much less pressure, they are no longer stable. There also used to be a bunch of ice on top of those rocks that melted fairly recently after the last ice age which only adds to that pressure. This particular event was probably also aided in some way by temperature changes from day to night.